1. Field of the Invention
The invention relates to a method for adjusting a speed-detection device for an object lens in an optical pickup head, and more particularly for adjusting the speed-detection device of the object lens under conditions of accelerating or decelerating optical pickup heads.
2. Description of the Related Art
In an optical disc drive, the object lens is positioned in the optical pickup head by an elastic means, such as a four-wire actuator. As the rotation speed of the optical disc drive becomes higher, and the requirement of seek-time becomes shorter, the object lens on the optical pickup head vibrates and shifts easily while tracking on the track. Therefore, the object lens and the optical pickup head experience relative motion by the shifting and vibrating phenomena. The optical pickup head is further equipped with a position-detector for detecting the location of the object lens relative to the optical pickup head, and a feedback circuit transforms the location of the object lens into a speed signal for the object lens so as to control the motion.
However, each optical disc drive does not have the same characteristics of mechanism, position-detector and feedback circuit, and the speed-control loop for controlling the position of the object lens in different optical disc drives output different gains. The position-detector, feedback circuit, and a programmable gain amplifier constitute a speed-detection device. Before the optical disc drives reach market, the speed-detection device must be adjusted. A conventional method for adjusting the speed-detection device uses a testing disc with predetermined eccentric value. When the optical disc drive tracks on the tracks on the testing disc, the object lens in the optical pickup head moves in a simple harmonic motion. If the eccentric value of the testing disc is A and the angular velocity of the testing disc is ω, the maximum speed of the object lens is Vmax=ω×A according to the simple harmonic motion. While each unadjusted optical disc drive conventionally uses the same testing disc, the object lens has the same maximum speed. Therefore, the conventional method adjusts the programmable gain amplifier in the speed-detection device, so that the maximum value of the speed signal of the object lens output from each speed-control device are the same.
However, the conventional method has to consider the relative positions of the eccentric axis and the spindle axis. We have to do the adjustment at several different positions and take the average value to get an accuracy result. The conventional method takes too much time to accomplish.